Resinous compositions which have so far been used for permanent protective masks, such as solder resist, chemical galvanizing resist, etc., in the printed circuit industry are those containing a heat-curable resin such as an epoxy resin, a melamine resin or the like as a main component, and pattern formation using them have been performed frequently by the screen printing process. A primary end of solder resists consists in making a barrier to the soldering region to prevent the generation of solder bridges and the corrosion to conductors, and further to retain electric insulation between conductors over a long period of time.
However, with the recent advances of high density mounting, such as very large scale integration (LSI) or the like, a reduction of the space between conductors have been required, severe demands have been imposed on electric insulation between conductors, and excellent dimensional precision has been required of solder resist and the like. On the other hand, the screen printing process has essentially low resolving power, and is subject to various phenomena, such as blur and pinholes (in case of highly viscous ink), or bleeding, blot and slack (in case of ink with low viscosity). Therefore, the screen printing process has been unable to cope with a sharp increase in mounting density of a printed circuit boards.
Under these circumstances, light-sensitive resinous compositions which enable the pattern formation using photography (the image formation through development subsequent to imagewise exposure), have high sensitivity and high resolution, and are excellent in not only adhesiveness to a substrate, but also electrical and mechanical characteristics when made into dry films by curing, are watched with keen interest.
Also, light-sensitive resinous compositions of the type which can be developed with an alkaline aqueous solution or water have recently been desired from the standpoints of the betterment of the working environment, the reduction in the corruption of water quality, and so on.
Dry film type or liquid-state, developable, light-sensitive, resinous compositions have been developed as permanent protective masks for printed circuit boards to have patterns formed in accordance with photography.
As light-sensitive resinous compositions of the dry film type, for instance, JP-A-No. 57-55914 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (corresponding to U.S. Pat. No. 4,499,163) discloses films comprising urethanedi(meth)acrylate(s), linear high molecular compounds and photosensitizers, and JP-A-No. 62-247353 discloses films comprising specified novolak type epoxy resins modified by (meth)acrylic compound(s), and photosensitizers.
In general, however, light-sensitive resinous compositions of the dry film type tend to generate bubbles upon hot press adhesion, and are questionable as to heat resistivity and adhesiveness. Although JP-A-No. 52-52703 (corresponding to U.S. Pat. No. 4,101,364) discloses a special process including a step of performing hot press adhesion under reduced pressure with the intention of solving those problems, even this process was unable to ensure complete heat resistivity and adhesiveness.
On the other hand, a developable, light-sensitive, liquid, resinous composition is coated directly on a printed circuit board immediately before use as it is in liquid state, so the number of steps is less. In addition, because the compositions is liquid, it can form an image of high quality on a printed circuit board even when its surface is rugged, and is best suited, e.g., for a light-sensitive resinous composition, to form a permanent protective mask for a printed circuit board having a narrow space between conductors.
However, the liquid, light-sensitive, resinous composition of this type also has a defect that when exposure is carried out after the coating on a printed circuit board with pattern mask direct contact with the surface of the film formed, the pattern mask is stained.
For the purpose of obviating the stain problem of the pattern mask, there has been proposed a special process, as disclosed, for example, in JP-A-No. 57-164595, in which a pattern mask is arranged so as to leave a space between the pattern mask and a coated, but undried, light-sensitive resinous compositions, the composition is cured by optical exposure, and the uncured portion of the composition is removed. This is different from a general process comprising coating a liquid light-sensitive resinous composition on a printed circuit board and drying the composition to make into a dry film. However, this wet process also has difficulties in that since there is a need to leave a space between the coated liquid light-sensitive composition and the pattern mask because the composition is not first made into a dry film, the resolution becomes so much the worse, the process costs much because a special equipment is required, and so on.
Still another process of producing solder resist is disclosed, for example, in JP-A-No. 58-24144 and JP-A-No. 59-2049, in which a liquid light-sensitive resinous composition is coated in a uniform thickness on an active rays transmitting, transparent flexible support or a pattern mask, and immediately thereafter the flexible support and the pattern mask is moved so as to bring the coated composition into a face-to-face contact with a printed circuit board, and the coated composition is pressed against the printed circuit board under a definite pressure, thus laminating the liquid light-sensitive resinous composition in a uniform thickness, and then exposure is carried out and the flexible support or the pattern mask is peeled apart, followed by development. According to this process, there are brought up some questions that since air is caught in the laminate due to unevenness of the printed circuit board when the liquid light-sensitive resinous composition coated uniformly on the transparent flexible support or the pattern mask is pressed against the printed circuit board, resulting in the formation of bubbles in no small numbers inside the developed image, there is precariousness in heat resistance and adhesiveness, this process cost much because of a special equipment required, and so on.
A further process of preparing solder resist is disclosed in JP-A-No. 61-102652 and JP-A-No. 62-27736, which comprises coating a liquid light-sensitive composition on a printed circuit board, pressing a transparent flexible support or a pattern mask against the coated composition under a definite pressure, exposing them through the pattern mask, peeling apart the flexible support or the pattern mask, and developing the exposed coat. In this process also, special equipment for pressing the flexible support or the pattern mask under a definite pressure and exposing it to light is required, raising expenses.
Moreover, the above-described processes of exposing a liquid light-sensitive composition to light following a resist pattern as it is without drying have a problem that they are inferior to general ones in characteristics which they can ensure to the resulting solder resist, e.g., acid resistance, chemical resistance and so on, because the liquid light-sensitive composition uses a liquid prepolymer containing a great number of reactive monomers.
As examples of liquid light-sensitive resinous compositions to be employed for the solder resist formation process in which a liquid light-sensitive resin is coated, dried by heating, made to stand in close contact with a pattern mask, exposed to light, and developed, mention may be made of a liquid resist ink composition as disclosed in JP-A-No. 61-243869, which contains a compound obtained by making a novolak type epoxy resin, an unsaturated monocarboxylic acid and a polybasic acid anhydride to react with one another, and a liquid resinous composition as disclosed in JP-A-No. 62-187722, which contains as a main component a bisphenol type epoxy resin having unsaturated groups and carboxyl groups.
However, those liquid light-sensitive resinous compositions have disadvantages that since the photopolymerizing compounds obtained by making the epoxy resins contained as a main component to undergo the reaction are lower in sensitivity than ordinary photopolymerizable monomers, it takes a long time to effect exposure, so the working efficiency is lowered, or the photomask is damaged due to a rise in temperature during the exposure step.